The present invention relates to a system as defined in the preamble of claim 1.
As is known, homogenization of a biofuel, such as reduction of its moisture content, equalization of vapor tension differences of different organic compounds and reduction of particle size, promotes the combustion of the fuel when burned, increases steam production in a boiler and reduces the amount of waste gas emissions produced in the combustion process. Dried solid wood material produced by a drying process using e.g. a flue gas drier or a vacuum drier allows wood material not fit for use in the production process of a pulp and paper mill to be utilized in energy production instead of being transported to a dump area. Thus, dumping costs are reduced, and so are nitrogen emissions into the atmosphere from waste transporting vehicles using fossil fuels. Moreover, fluidized bed combustion does not require the use of auxiliary fossil fuels as are otherwise needed for the combustion of damp fuels, or the amount of these fossil auxiliary fuels is substantially reduced as the wood-based fuel has been dried and burns without problems.
However, previously known drying systems, i.e. flue gas driers and vacuum driers, have certain drawbacks. So far, the main purpose of the drying and combustion of damp biomass has been to get rid of damp mass that cannot be used as raw material for anything. Thus, damp mass has been dried using various kinds of waste heat, such as flue gases, obtained from different processes, without properly considering the effect of the fuel on the process as a whole. For instance, drying processes may use large amounts of warm air which is blown out into outer air in a humid state. Thus, both solid and gaseous impurities, odors as well as organic or solid compounds are emitted into the atmosphere from the drying process.
The object of the invention is to eliminate the problems referred to above. A specific object of the invention is to disclose a new type of system that will allow a more effective utilization of a damp biofuel as well as a definite reduction in the amount of emissions into the environment as compared with prior-art solutions.
As for the features characteristic of the invention, reference is made to the claims.
The system of the invention comprises a boiler, preferably a fluidized bed boiler, in which a biofuel is burned in order to recover and utilize the energy contained in it. The system of the invention is based on multi-stage drying, i.e. at least two successive separate heat drying chambers and drying stages. Thus, according to the invention, the system comprises a first heat drying chamber, into which a flow of drying gas is passed and which is also provided with a fuel supply for supplying a fuel to be dried into the first heat drying chamber. In addition, the system comprises at least a second heat drying chamber and an intermediate heating unit, the latter being used to heat the flow of drying gas between the heat drying chambers. The system also comprises an intermediate supply for passing the fuel from the first heat drying chamber into the second heat drying chamber. Thus, the system of the invention has at least two and preferably more than two separate heat drying chambers in series, i.e. in cascade so that substantially the same drying gas flow is heated during each passage between chambers. In addition, the system of the invention comprises a boiler supply for passing the fuel from the last heat drying chamber into a boiler, and an outlet for passing the flow of drying gas from the last heat drying chamber into the boiler, preferably into different combustion zones in the boiler.
In an embodiment of the invention, the drying gas flow is also cooled between the heat drying chambers, thus allowing it to be dehumidified before being heated.
The system of the invention is based on the fundamental idea that the higher the temperature of the drying gas flowing into a drying stage, the smaller is the volume flow of drying gas needed. Thus, the smaller the volume flow of the drying gas supplied into the heat drying chamber, the easier will it be to conduct the more humid gas flow after the drying stage into a fluidized bed boiler where it is to be thermally oxidized. Likewise, the higher the temperature of the drying gases supplied and the lower the moisture content of the fuel supplied into the stage, the higher is the internal temperature within the drier. Thus, in the system of the invention, preliminary and intermediate heating stages are used to minimize the drying gas flows and to enable their effective thermal treatment in the boiler.
Similarly, in the system of the invention, the higher the temperature of the drying gases supplied into individual heat drying stages, the larger is the amount of organic compounds evaporated in consequence of steam distillation from the fuel being dried. Therefore, the gases leaving the drying stage also have a certain thermal value in combustion. As a result of the multi-stage preliminary and intermediate heating of the drying gas flow, the water-binding capacity, i.e. the adiabatic water-binding capacity of the drying gases is increased as compared with passing hot drying gases of 100-500xc2x0 C. into a single-stage fuel drier. This is part of the reason behind the fact that the higher the temperature of the drying gases supplied into the drier, the more is the volume flow needed in the drier reduced.
The drying gas flow used in the system of the invention may consist of combustion gases, air heated by combustion gases or a suitable mixture of combustion gases and air. A mixture of combustion gas and air is advantageous because it dilutes the oxygen content of the drying gas leaving the last drying stage. This makes it easier to create under-stoichiometric conditions with respect to oxygen of the combustion air in the fluidized bed of the fluidized bed boiler burning the dry fuel.
The system preferably comprises a pre-heating unit for pre-heating of the drying gas flow before the first heat drying chamber. The pre-heating unit may consist of a unit in which air is heated by combustion gases or it may be a unit in which relatively hot combustion gases or mixture of combustion gases and air are/is heated further using e.g. bled steam.
In a preferred case, the pressure in one or more drying stages, e.g. in the first drying stage, is regulated or is maintained at a given level in relation to the atmospheric pressure. Preferably a pressure below atmospheric is used, but normal atmospheric pressure and a pressure above atmospheric are also possible in some cases, depending on the quality and moisture content of the fuel to be treated.
In an embodiment of the invention, the system comprises a fuel pre-heating unit disposed before the first heat drying chamber. Thus, the fuel can be pre-heated and pre-dried at a relatively low temperature, e.g. 50-80xc2x0 C., before the actual heat drying process. For such low-temperature pre-heating and pre-drying, it is possible to use any flow of exhaust heat released from the process or otherwise difficult to utilize. The use of a fuel pre-heating unit is almost always profitable because the process generally produces various secondary energy flows that can be used to raise the temperature of the fuel and reduce its moisture content without substantial additional energy costs.
In an embodiment of the invention, the drying gas flow coming out of a heat drying chamber comprises an intermediate outlet placed before an intermediate heating unit, said outlet serving to remove a portion of the relatively humid gas flow from the drying circulation. Depending on the temperature and moisture content of the gas flow portion to be removed and on the amount of organic compounds contained in it, said gas flow portion can be passed either into outer air, into the boiler for use in combustion or into a pre-heating unit for recovery of the heat contained in it.
In the drying gas flow, it is further possible to use various separators, e.g. a cyclone, for removing e.g. solid particles and moisture in the form of an aerosol from the drying gas flow, in addition to the possibility of reducing moisture by cooling the flow as described above. Separators are preferably used after each drying stage. It is also possible to treat the drying gas flow in a condensing scrubber to remove extra moisture from the gas flow before it is passed into the boiler. The condensed water can then be passed into the wastewater treatment system of the plant.
The boiler used in the system of the invention is preferably a known type of fluidized bed boiler into which humid drying gases produced in the system can be easily passed for combustion. As drying apparatus, it is possible to use solid bed, fluidized bed or circulating mass drier applications. The system of the invention uses two or more driers connected in series, i.e. in cascade, their number depending on the operating environment in question and the drying results aimed at. The capacity of the system can be readily increased by using a parallel configuration of a required number of series connected drying apparatus in themselves corresponding to the system described above.
As compared with prior art, the system of the invention provides significant advantages. The volume flows of the required drying gases are small as compared with prior-art solutions, allowing their adiabatic water-binding capacity to be significantly improved via preliminary and intermediate heating. Similarly, due to their small volume, said flows can be easily fitted in different stages among the combustion air passed into a fluidized bed boiler. However, the drying gases are preferably not passed directly into the boiler furnace to avoid energy losses; instead, they are supplied into boiler areas where the combustion gases have a temperature of the order of 750-800xc2x0 C., which is sufficient for thermally oxidizing the organic compounds contained in the drying gases, producing carbon dioxide.
Although the exhaust gases from the system for drying a damp fuel are taken into the boiler and thermally oxidized, which significantly reduces the combustion gas emissions, the amount of fresh steam produced in the boiler is larger than the corresponding values for a mere boiler using damp fuel without a system according to the invention for drying damp fuel outside the furnace.
The increase in the thermal value of the fuel achieved by the system of the invention is sufficient to compensate for the energy consumed in the treatment of the drying gases (pre-heating, intermediate heating and heating of the drying gases inside the boiler to the combustion temperature). A further advantage is that the drying gases need not be taken to boiler furnace areas where the temperature exceeds 750-800xc2x0 C. because in this way the system avoids losing too much of the energy of the combustion gases which has to be utilized for steam production in the boiler.
Thus, in-the system of the invention, the net energy production in fluidized bed combustion is increased, combustion gas emissions are decreased and condensate emissions are minimized when the minimum temperatures of the drying gases flowing out from different drying stages of the combustion gas drier are in the range of 95-100xc2x0 C.
The multi-stage drying system of the invention is applicable for use in conjunction with boilers of different categories regarding fuel efficiency, including both small plants and plants of over 100 MW. However, the increase in the net combustion efficiency achieved by the drying system described is the greater the larger is the power plant boiler and the lower is the fuel dampness value aimed at.
Using the system of the invention for immediate drying of a damp fuel, the period of storage of the fuel is shortened and the loss of its thermal value due to rotting is avoided. In addition, when a fluidized bed boiler is operated at net energy production levels corresponding to those achieved earlier by burning damp fuel, the mass flow of damp fuel at the input is reduced, which is of great importance in reducing the emissions from the combustion process.
As there are generally large variations in the quality and dampness values of different biofuels, the system of the invention provides the advantage that the multi-stage drying process, being additionally easy to regulate, balances these variations, permitting smooth operation of the boiler.